2. Contents Basic concepts Assumptions of sintering theories Coarsening and densification Viscous flow Evaporation-condensation Surface diffusion Volume diffusion Grain boundary diffusion Plastic flow Overview of mass transport processes Adhesion, rearrangement, and repacking Initial-stage neck growth Intermediate stage Final stage Coarsening versus densification Theoretical introduction Sintering stress Stages of sintering Data analysis Calculations of sintering rates Sintering diagrams Summary Mass transport mechanisms
3. Basic Concepts Sintering forms solid bonds between particles when they are heated. ∆ ( γ A) = ∆ γ ∙ A + γ ∙ ∆ A ∆ γ ∙ A Densification γ ∙ ∆A Coarsening ∆ ( γ A) Densification and Coarsening
4. The temperature needed to induce sinter bonding versus densification depends on the material and particle size . Most materials exhibit sintering at homologous sintering temperatures between 0.5 and0.8. With higher temperatures, longer times, or smaller particles, the bond grows more rapidly and densification becomes evident.
5. Adhesion Initial Intermediate Final This stage occurs when particles come into contact. A weak cohesive bond Rapid growth of the interparticle neck. The pore structure becomes smooth and develops an interconnected. Giving a larger average grain size with fewer grains. The pores are spherical and closed, grain growth is evident.
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8. Coarsening and Densification Sintering of Boron ( coarsening with out densification ) It is common to observe sintering with simultaneous densification and coarsening.
9. Sintering stress The stress from interfacial energies acting over curved surfaces in a sintering system is termed the sintering stress. : The Laplace equation For a small neck, the gradient can be quite large. Thus, the stress gradient provides a driving force for mass flow to the neck. As the neck grows, the curvature gradient is relaxed and the proess slows.
10. MASS TRANSPORT MECHANISMS Surface transport (no shrinkage or densification) Evaporation-Condensation Surface diffusion Volume diffusion Bulk transport (shrinkage or densification) Plastic flow Grain boundary diffusion Volume diffusion Viscous flow low-temperature Low-stability Only for compacted powders, where the initial dislocation density is high. Most crystalline materials Amorphous materials
11. Viscous Flow Over a limited temperature range, an amorphous material has a viscosity that varies with a temperature dependence . Q=activation energy η 0 =proportionality coefficient T=absolute temperature k=boltzmann’s constant Early during isothermal sintering the neck diameter X between particles of diameter D grows in proportion to the square root of the sintering time. γ =surface energy t=time Since higher temperatures lower the viscosity, there is a progressive increase in neck size with temperature.
12. Sintering data for glass spheres, where the square Amorphous materials is nonexistent a grain boundary at the sinter bond. Consequently, as neck growth proceeds, amorphous materials can readily achieve a zero curvature condition where the convex and concave radii are equal but opposite in sign.
13. Evaporation-Condensation Vapor transport during sintering leads to the repositioning of atoms located on the particle surface, without densification . P=equilibrium vapor pressure T=temperature P 0 =material constant Q=activation energy for evaporation k=boltzmann’s constant Higher temperatures give a higher vapor-pressure and more vapor-phase transport, since the flux depends on the evaporation rate. Consequently, evaporation occurs preferentially from flat or convex particle surfaces. Preferential deposition occurs at concave necks between particles where the vapor pressure is slightly below equilibrium. For many materials evaporation-condensation transport is slow at typical sintering temperatures. Arrhenius’s equation
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15. Once the neck size reaches an equilibrium dictated by the solid-vapor dihedral angle, further neck growth depends on grain growth. In final-stage sintering, closed pores become distorted by pore migration with the moving grain boundaries.
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17. Volume Diffusion Volume diffusion adhesion From the neck surface through the particle interior, with subsequent emergence at the particle surface. (no densification or shrinkage) Volume diffusion densification It involves vacancy flow to the interparticle grain boundary from the neck surface. Dislocations and vacancies Vacancies can be emitted or annihilated by dislocations via a process termed dislocation climb.
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19. Grain Boundary Diffusion Grain boundary diffusion is relatively important to the sintering densification of most metals and many compounds. A sketch that visualizes the repeated defect Structure associated with a grain boundary Having a 36.9° misorientation between grains. As sintering progresses, transport takes place between pores via the grain boundary, leading to pore coarsening. Vacancy accumulation on a grain boundary requires motion of the boundary, and this is resisted by contacting neighbors. It is a high grain boundary energy that is a prime cause of simultaneous grain growth during sintering. -> segregates to grain boundary (to add a species, Ni,Fe, W, Mo,Fe,Cu…)
20. Plastic Flow Plastic flow is the motion of dislocations under stress. The dislocation flow is restricted to the early stage of sintering. : As the neck enlarges, the shear stress declines and falls below the flow stress for the material and the process becomes inactive. Siegel Dislocation participate in sintering during heating, especially if the powders was subjected to plastic deformation during compaction. Schatt and co-workers Demonstrated densification rate improvements because of dislocation climb with the rate of pore elimination.
23. Initial-Stage Neck Growth The initial stage ends when the necks begin to impinge at approximately a neck size ratio X/D of 0.3. Assume that sintering occurs between two equal spheres with conservation of volume by a single mass transport mechanism. Here is a gradient in the curvature over the sintering geometry. The curvature gradient drives the mass flow, by any of the mechanisms discussed above, to smooth the surface and possibly densify the structure. Consider surface-transport-controlled sintering where at any point on the surface curve, The instantaneous change in the neck profile, At the point v on the neck profile the principal radii of curvature The local curvature κ ,
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25. Shrinkage Shrinkage during initial stage sintering follows a kinetic law. The parameter B is exponentially dependent on temperature Surface area Surface-area-reduction kinetics of the sintering of 0.2μm Alumina at 750 ℃.
26. Intermediate Stage Simultaneous pore rounding Dinsification Grain growth The driving force is elimination of the remaining surface energy. pore structure Retarded grain growth & enhanced diffusion: temperature, microstructure Grain growth becomes increasingly active as the Pore structure collapses. The pinning effect of the pores diminishes as they Shrink and occupy less grain boundary area. -> coasening :Smaller grains aid densification Densification rate
28. Grain growth forces the grain boundary to be curved, leading to a progressive increase in grain boundary area as the grain boundary bows to sustain contact with a slower-moving pore. A critical condition is achieved where it is favorable for the boundary to break away from the pore. grain growth rate > pore mobility : isolation pore (slow densification by long-range volume diffusion) grain growth rate < pore mobility : continue to shrink (surface diffusion or evaporation-condensation)
29. Usually, the grain boundary mobility is much larger than the pore mobility, leading to pore drag, which reduces the rate of grain growth during final-stage sintering, as long as the pores remain attached to the grain boundaries . pores motion : surface diffusion grain boundary motion : temperature, grain size and grain boundary energy The conditions where pores remain attached to grain boundaries during final-stage sintering : F p =the force on a pore which varies with solid-vapor surface energy divided by the pore size F G =the force on the grain boundary, varies with the curvature of the grain boundary N=parameter, varies with the inverse square of the pore spacing
30. Rate of grain growth : K f =a geometric constant, relates the pore spacing and the grain boundary curvature (≈1) For the typical case of pore motion by surface diffusion, a relative coarsening to densification ratio Γ : D S =surface diffusivity D B =grain boundary diffusivity γ SS =grain boundary energy γ SV =solid-vapor surface energy Γ < 1 : full density
31. Doped MgO : the coarsening to assist in densification Doped ZrO2 : assists densification by inhibiting grain growth Rapid grain growth is observed once the Grain boundaries break away from the pores.
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33. Avoiding the conditions that give boundary separation from the pores is essential for high sintered densities.
34. DATA ANALYSIS Slope = volume diffusion control Time exponent exactly matches viscous flow control The more rapid sintering at the higher temperatures. Arrhenius plot Slope = 405kJ/mol;activation energy
37. SINTERING DIAGRAMS A sintering diagram proves useful in condensing and representing sintering behavior. Indicates the dominant transport mechanism This plot shows the neck size ratio versus isothermal sintering temperature for four hold times.
38. The sintering diagram based on density for submicrometer alumina with four time lines. A plot of fractional density versus temperature for isothermal sintering of 6 μ m tungsten with two time lines.
